Investigation of the resonance-assisted hydrogen bond in model β-diketones through localized molecular orbital analysis of the spin-spin coupling constants related to the O-H···O hydrogen bond

Covalence and π-electron delocalization influence on hydrogen bonds in proton transfer process of o-hydroxy aryl Schiff bases: A combined NMR and QTAIM analysis

Within the framework of the density functional theory approach, we studied the relationship between the chemical nature of intramolecular hydrogen bonds (HBs) and nuclear magnetic resonance (NMR) parameters, J-couplings and ¹H-chemical shifts [δ(¹H)], of the atoms involved in such bonds in o-hydroxyaryl Schiff bases during the proton transfer process. For the first time, the shape of the dependence of the degree of covalence in HBs on ¹J(N-H), ¹J(O-H), ^2hJ(O-N), and δ(¹H) during the proton transfer process in o-hydroxyaryl Schiff bases was analyzed. Parameters obtained from Bader's theory of atoms in molecules were used to assess the dependence of covalent character in HBs with both the NMR properties. The influence of π-electronic delocalization on ^2hJ(N-O) under the proton transfer process was investigated. ^2hJ(O-N) in a Mannich base was also studied in order to compare the results with an unsaturated system. In addition, substituent effects on the phenolic ring were investigated. Our results indicate that the covalent character of HBs on both sides of the transition state undergoes a smooth exponential increase as the δ(¹H) moves downfield. The degree of covalence of the N⋯H (O⋯H) bond increases linearly as ¹J(N-H) (¹J(O-H)) becomes more negative, even after reaching the transition state. Non-vanishing values of spin dipolar (SD) and paramagnetic spin orbital terms of ^2hJ(O-N) show that π-electronic delocalization has a non-negligible effect on tautomeric equilibrium and gives evidence of the presence of the resonance assisted HB.Variation of the SD term of ^2hJ(O-N) follows a similar pattern as the change in the para-delocalization aromaticity index of the chelate ring.

Accurate ab initio calculations of O-HO and O-H(-)O proton chemical shifts: towards elucidation of the nature of the hydrogen bond and prediction of hydrogen bond distances

The inability to determine precisely the location of labile protons in X-ray molecular structures has been a key barrier to progress in many areas of molecular sciences. We report an approach for predicting hydrogen bond distances beyond the limits of X-ray crystallography based on accurate ab initio calculations of O-HO proton chemical shifts, using a combination of DFT and contactor-like polarizable continuum model (PCM). Very good linear correlation between experimental and computed (at the GIAO/B3LYP/6-311++G(2d,p) level of theory) chemical shifts were obtained with a large set of 43 compounds in CHCl3 exhibiting intramolecular O-HO and intermolecular and intramolecular ionic O-H(-)O hydrogen bonds. The calculated OH chemical shifts exhibit a strong linear dependence on the computed (O)HO hydrogen bond length, in the region of 1.24 to 1.85 Å, of -19.8 ppm Å(-1) and -20.49 ppm Å(-1) with optimization of the structures at the M06-2X/6-31+G(d) and B3LYP/6-31+G(d) level of theory, respectively. A Natural Bond Orbitals (NBO) analysis demonstrates a very good linear correlation between the calculated (1)H chemical shifts and (i) the second-order perturbation stabilization energies, corresponding to charge transfer between the oxygen lone pairs and σ antibonding orbital and (ii) Wiberg bond order of the O-HO and O-H(-)O hydrogen bond. Accurate ab initio calculations of O-HO and O-H(-)O (1)H chemical shifts can provide improved structural and electronic description of hydrogen bonding and a highly accurate measure of distances of short and strong hydrogen bonds.